The present invention relates to PBX systems and, more particularly, to digital PBX systems in which a plurality of signals are pulse-code-modulated for transmission on a data bus.
A conventional digital PBX system such as, for example, one disclosed in U.S. Pat. No. 4,069,399, employs at least one bus carrying a plurality of multiplexed digital data signals for delivery to, and/or for reception from, a plurality of terminals (telephone instruments, data sets, central office trunks, T1 carrier facilities). The data on the bus is sorted by time and space switches in a plurality of network modules or loops to yield a plurality of data streams. Each data stream is made up of a first plurality of time slots, conventionally 32 time slots in a frame, and a second plurality of frames, conventionally 8 frames, making up a main frame. A data word is an 8-bit word consisting of one bit from a corresponding time slot in each of the 8 frames making up a main frame. Normally, one of the 32 time slots is devoted to signalling information, and a second one is reserved, thus leaving 30 time slots for the transmission of data.
The interface between the network and its terminals takes place in line cards. Each line card is capable of serving from one to about 8 terminals. The total number of terminals that can be served by a loop depends on the quality of service that is desired, as is described below.
A conventional measure of throughput in a PBX system is in hundreds of channel seconds per hour, usually abbreviated CCS. Each channel provides 36 CCS (there are 3600 seconds per hour). The loop is capable of providing 36 CCS times the number of time slots available for communication in the loop. In the case of a loop having 30 time slots available (32 time slots less one signalling time slot and one reserved time slot), the loop capacity is 36.times.30=1080 CCS. If fully non-blocking operation is desired, then each terminal requires 36 CCS. In such a case, only 30 terminals can be accommodated. In fact, however, many applications require far less than 36 CCS. In motel/hotel use, for example, a typical terminal may require only 2 or 3 CCS. Other applications may require channels using anywhere from one or two to 36 CCS. Accordingly, it is possible for one conventional loop to serve many more than 30 terminals.
If somewhat less than full non-blocking operation is required, some degradation in service is anticipated since there is a probability that communications may be desired when no channel capacity remains available. Service degradation can be predicted with some degree of accuracy, depending on the applications of the particular terminals. As long as it is infrequent enough to be no more than a minor inconvenience, such service degradation can be tolerated. It is thus possible, and indeed is conventional, to continue to add terminals to a loop until a prediction of service degradation reaches a predetermined level.
A measure of grade of service (GOS) degradation is the probability of blocking. For a probability of blocking of about 0.01, the 30 channels of a conventional loop provide a total capacity of about 660 CCS. It will be noted that the total capacity is decreased from 1080 to about 660 CCS, in return for which the number of terminals which can be serviced is increased by a factor that depends on the type of service required on the terminals. In some applications, the number of terminals exceeds the number of channels (or available time slots) by a factor of 8.
In practice, line cards are packaged in peripheral equipment shelves. Each peripheral equipment shelf is capable of containing a predetermined maximum number of line cards such as, for example, 10 or 16. All line cards in a peripheral equipment shelf communicate with the same network loop. One network loop may communicate with the line cards in two or more peripheral equipment shelves. For economies in peripheral equipment shelves, as well as space and power conservation, it is desirable to employ substantially all of the capacity of peripheral equipment shelves.
The design of prior-art equipment contains no provision for matching the traffic capacity of its network loops (called loop capacity or channel capacity) with the needs of the terminals serviced by line cards in a peripheral equipment shelf. Normally, line cards are added to a peripheral equipment shelf until the loop capacity is utilized by an amount determined by the grade of service selected. If the loop capacity is not fully absorbed by the terminals connected to the line cards, a decision must be made whether to add a second peripheral equipment shelf to hold additional line cards to absorb the remainder of the loop capacity. If the remaining channel capacity is less than that which can be absorbed by terminals connected to a second full set of line cards in a second peripheral equipment shelf, then the decision entails either under-utilizing a peripheral equipment shelf, or under-utilizing channel capacity. If the second peripheral equipment shelf is supplied, then less than all of its full complement of line cards is required, thus under-utilizing the peripheral equipment shelf and increasing the usage of space and electricity. If the second peripheral equipment shelf is omitted, then the remaining unused channel capacity of the network loop is wasted. Neither of these alternatives is desirable.
A large PBX system, such as disclosed in the referenced patent, includes a plurality of network loops. In the prior art, each network loop is served by its own set of one or more peripheral equipment shelves. Thus, the inefficiencies in under-utilized peripheral equipment shelves or under-utilized channel capacity outlined above are multiplied by the number of network loops. The total number of unused areas in peripheral equipment shelves, and/or the total unused channel capacity due to decisions omitting additional partly utilized peripheral equipment shelves, can result in a significant reduction in PBX system performance.